function A0_coef = RK2(A0_coef,dt,N,M,D,B,C)
New_A0 = one_step(A0_coef,N,M,D,B,C);
New_A0_coef = A0_coef + dt*New_A0;
New_A0 = one_step(New_A0_coef,N,M,D,B,C);
A0_coef = 0.5*A0_coef + 0.5*New_A0_coef + 0.5*dt*New_A0;
end